The present invention relates to an improved phenolic urethane binder composition to bind foundry cores and molds. The present invention also relates to a method for improving the strength of foundry cores and molds made using such a binder and in particular the humidity resistance of such cores and molds. This invention further relates to the reaction product of an acidic fluoride compound, and silicon dioxide useful in phenolic urethane foundry binder compositions for the improvement of humidity resistance.
Phenolic urethane binders or binder systems for foundry cores and molds are known. In the foundry art, cores or molds for making metal castings are normally prepared from a mixture of an aggregate material, such as sand, and a binding amount of a binder or binder system. Typically, after the aggregate material and binder have been mixed, the resulting mixture is rammed, blown or otherwise formed to the desired shape or pattern of the core or mold, and then cured to a solid.
Generally, resin binders used in the production of foundry molds and cores may be cured at high temperatures to achieve the fast-curing cycles required in foundries. However, resin binders have been developed which cure at low temperatures. These processes are preferred over high-temperature curing operations that have higher energy requirements and often emit undesirable fumes. Also, these processes offer productivity advantages over the high-temperature curing operations.
One group of processes which do not require heating in order to achieve curing of the resin binder are referred to as phenolic urethane nobake processes. In such processes, the binder components are coated on the aggregate material, such as sand, and the resulting mixture is rammed, blown or otherwise formed to the desired shape or pattern, either a core or mold. Curing of the binder is achieved without heating. In these processes, the binder components typically include a part 1 binder component, a part 2 binder component and a liquid catalyst.
Another process, which does not require the application of heat to cure a core or mold, is the cold box process. In this process, a foundry core or mold is prepared by mixing sand with a two component binder, discharging the mixture into a pattern, and curing the mixture by contacting the binder with a vaporous catalyst.
As alluded to above, the binder for the urethane cold-box or nobake systems is a two-part composition. The part 1 component of the binder is a polyol (comprising preferably hydroxy containing phenol-formaldehyde resins) and the part 2 component is an isocyanate (comprising preferably polyaryl polyisocyanates). Both parts are in a liquid form and are generally used in combination with organic solvents. To form the binder and thus the foundry sand mixture, the part 1 component and the part 2 component are combined. After a uniform mixture of the foundry sand and parts 1 and 2 is achieved, the foundry mix is formed or shaped as desired. Parts 1 and/or 2 may contain additional components such as, for example, mold release agents, plasticizers, inhibitors, and the like.
Liquid amine catalysts and metallic catalysts, known in the urethane art, are employed in a no-bake composition. The catalyst may be incorporated into either the part 1 component or the part 2 component of the binder or it may be added after uniform mixing as a third part. By selection of a proper catalyst, conditions of the core making process, for example, worktime and strip time, can be adjusted.
In cold box technology, the curing step is accomplished by suspending a tertiary amine catalyst in an inert gas stream and passing the gas stream containing the tertiary amine, under sufficient pressure to penetrate the molded shape until the resin is cured.
Improvements in resinous binder systems which can be processed according to the cold box or nobake process generally arise by modifying the binder components, i.e., either the polyol part or the isocyanate part. For instance, U.S. Pat. No. 4,546,124, which is incorporated herein by reference, describes an alkoxy modified phenolic resin as the polyhydroxy component. The modified phenolic resin improves the hot strength of the binder systems. U.S. Pat. No. 5,189,079, which is herein incorporated by reference, discloses the use of a modified resole resin. These resins are desired because they emit reduced amounts of formaldehyde. U.S. Pat. No. 4,293,480, herein incorporated by reference, relates to improvements in the isocyanate component, which enhances shake-out properties of non-ferrous castings.
One of the shortcomings of the phenolic urethane cold box binders is that, under humid condition, specimens made with this type of binder system deteriorate substantially. Humidity is a concern because its effect is to reduce the tensile strength of produced cores. The presence of water or water vapor can react with any unreacted isocyanate, thus producing a weak, undesirable chemical structure. Also, the presence of excessive water or water vapor can cause a drop in tensile strength of cured prior art cores exposed to these conditions. The effect may even be insidious, as other more easily measured parameters such as cure time, may not be influenced, thus providing the user of a binder with a false sense of security. Hundreds of cores may be produced before the affects of humidity become apparent. Accordingly, the ability to improve humidity resistance is a significant advance in the art.
Fluoride, including hydrofluoric acid, modifications of resin binder systems are known. A range of benefits, such as faster cure speed, humidity resistance, improved collapsibility, have been reported. However, the use of hydrofluoric acid alone, for example, produced highly variable results with respect to improved humidity resistance. Accordingly, the use of hydrofluoric acid alone has long been considered undesirable.
It would therefore be an advantage to have a phenolic urethane binder system that provides significantly stronger cores and molds under high humidity conditions. It would be yet a further advantage to have a method for improving the humidity resistance of cores and molds bound with a phenolic urethane resin. It would be an even further advantage to provide an additive to phenolic urethane binders thereby providing additional humidity resistance to the cured resin.
Unexpectedly, in view of the foregoing difficulties, it has now been discovered that the tensile strength in cured cores and molds may be improved by using a new and improved phenolic urethane binder, or, as an alternative embodiment, a new and improved additive. In one embodiment, the new and improved phenolic urethane binder comprises a phenolic resole, hydrofluoric acid and an inorganic silicon compound. In another embodiment, the new and improved phenolic urethane binder comprises hydrofluoric acid and a boron compound. In yet other embodiments, the new and improved phenolic urethane binder includes, other silicon bearing compounds and other fluoride bearing acids. In an alternative embodiment of the present invention, an additive for improving the humidity resistance of foundry cores and molds comprises hydrofluoric acid and an inorganic silicon compound. In another embodiment, the additive comprises hydrofluoric acid and a boron compound.
A major advantage provided in accordance with the invention is that significantly stronger cores and molds are provided than were heretofore obtainable under high humidity conditions with prior art phenolic urethane binder systems. Another advantage provided by the present invention is that a new additive is provided that realizes the synergistic benefits from combining a fluoride bearing acid with a silicon or boron compound. A further advantage provided by the present invention is that cured foundry shaped articles having improved humidity resistance may be provided. Still another advantage is that a new and improved method for improving the humidity resistance of cores and molds with a phenolic urethane binder is provided.